Move mjcb_.* callbacks into mjModel

### The feature, motivation and pitch

Multiple different MuJoCo simulation models in one process are not cleanly separated
because the installed `mjcb_.*` callbacks from `mujoco.h` are global.

Please move these function pointers into the `mjModel` struct to provide separation
between different simulation contexts. All callbacks except `mjcb_time` already require a pointer to a `mjModel` struct so this change should not be too invasive.

### Alternatives

It is possible to swap the pointers before each call to `mj_step`.
However, this results in some kind of "Big Simulator Lock".

### Additional context

_No response_